In a keyed instrument, such as an electronic organ, certain music can be played by manipulating only the white keys, while other musical selections require manipulating the white and the black keys. Playing a selection in certain keys, for example C major, may require playing only the white keys. Playing that same selection in the key of D, requires the playing of both black and white keys. Accordingly, transposing from one musical key to another is difficult for the unskilled musician.
Many devices have been developed for transposing the key of an electronic instrument. These are simple in concept, but expensive and involved in implementation. For example, transpose switches may transpose tone signal outputs in respect to key switches, or all of the separate tone signal sources may be re-tuned to produce the required transposed tones, i.e., the C oscillator may be uptuned to C.music-sharp., etc.
In U.S. Pat. No. 3,824,325 to Obayashi et al, issued July 16, 1974, a transposition system is disclosed which employs a pair of octave generators, a fixed frequency master oscillator, and a simple transpose switch. However, this system, because it employs cascade frequency division in its octave generators, is relatively inaccurate in its tone frequencies. A more accurate type of top octave frequency divider can be employed, but this requires a master oscillator in the megacycle range, and the philosophy of the patent then becomes inapplicable.
A transposer employing a fixed frequency oscillator and a voltage controlled oscillator in a phase locked loop, the VCO supplying clock pulses directly to an octave generator and the output of any selected divider of the octave generator being compared with a frequency derived by division with a fixed ratio from a crystal controlled oscillator, is disclosed at Page 14 of an article published by General Electric Company, on Nov. 30, 1970, the author being Gerald L. Kmetz. A similar system is patented in U.S. Pat. No. 3,800,060 to Hallman, issued Mar. 26, 1974.
In accordance with the present invention, the divider output of an octave generator, selected by a manual switch according to the transposition desired, is compared with a reference filter which introduces a phase shift as a function of frequency. Comparison of the phase of the output of the filter with its input in an integrator produces a voltage which controls the output of a VCO, which supplies clock pulses to the octave generator. No clock pulse source other than the VCO is required, contrary to the above described prior art system. (Kmetz)
The present invention has for its objective the provision of a transposer which employs a single octave generator, of the type which requires a single high frequency voltage controlled clock, and no additional oscillator.